PROJECT SUMMARY/ABSTRACT Diploid germ cells are transformed into haploid gametes via the reductive division of meiosis. After meiotic DNA synthesis, germ cells enter meiotic prophase I, which distinguishes itself from mitotic prophase by two major characteristics. First, meiotic prophase I is dramatically longer than mitotic prophase. Second, meiotic prophase I encompasses an intricate series of chromosomal events that contribute to the establishment of the haploid state. The extended length of prophase I is critical for the completion of these chromosomal events, as premature exit from prophase I causes recombination defects and chromosome mis-segregation. In mammals, the retinoic acid-induced gene Stra8 is required for the initiation of meiosis and expression of the meiotic chromosomal program. However, the mechanism by which meiotic prophase I is prolonged in mammalian germ cells remains largely uncharacterized. Recent work from the Page lab suggests that Meioc may help establish the meiotic cell cycle program in mice. Meioc -/- germ cells initiate meiosis and the meiotic chromosomal program. However, they fail to complete meiotic prophase I and prematurely enter an abnormal metaphase. Several preliminary observations suggest that this phenotype is based on the failure to inhibit the mitotic cell cycle program, indicating that the meiotic cell cycle program is not properly established. This proposal will test the hypothesis that MEIOC represses the mitotic cell cycle program in germ cells, and thereby prevents precocious cell cycle progression from meiotic prophase I to metaphase. This hypothesis will be tested via three specific aims. First, the Meioc -/- phenotype will be more carefully characterized by identifying the first differences in transcript levels that occur between wild-type and Meioc -/- germ cells using RNA-seq. In addition, the transcripts with which MEIOC interacts will be identified via RNA immunoprecipitation and sequencing (RIP-seq). The second aim will determine how MEIOC molecularly affects the transcripts to which it binds by identifying the proteins that interact with MEIOC and by measuring changes in RNA stability between wild-type and Meioc -/- germ cells. Finally, the third aim will determine whether MEIOC regulates the cell cycle at meiotic initiation. First, Meioc transcript and MEIOC protein expression will be assayed to verify that they are expressed normally in male and female germ cells that fail to initiate meiosis due to the absence of Stra8. Then, Meioc -/-; Stra8 -/- germ cells will be examined to determine whether they continue to cycle mitotically, due to loss of MEIOC's repression of the mitotic cell cycle program. Collectively, these fundamental studies on MEIOC's regulation of the length of meiotic prophase I will provide key insights into the establishment and regulation of the meiotic cell cycle program, with broader implications for fertility and development.